1. Field of the Invention
The present invention relates to a method of driving a chopper DC/DC converter, and a DC/DC converter.
2. Description of the Related Art
There have heretofore been widely used DC/DC converter apparatus as switching power supplies having switching devices such as MOSFETs, IGBTs, or the like.
For example, there has been proposed a vehicle (hereinafter referred to as “electric vehicle”) incorporating a DC/DC converter apparatus for increasing and reducing a DC voltage, which is connected between an electricity storage device and a motor that is energized by an inverter. On the electric vehicle, when the motor is energized, the voltage across the electricity storage device is increased by the DC/DC converter apparatus and applied to the inverter, and when the motor regenerates electric power, the regenerated voltage from the inverter is lowered by the DC/DC converter apparatus and applied to charge the electricity storage device.
There has also been proposed a vehicle (hereinafter also referred to as “fuel cell vehicle”) which also uses a motor as a propulsive source. The fuel cell vehicle includes a fuel cell directly connected to the motor which is energized by an inverter. A DC/DC converter apparatus for increasing and reducing a DC voltage is connected between an electricity storage device and the junction between the fuel cell and the motor. The fuel cell is used as a main power supply, and the electricity storage device as an auxiliary power supply for assisting the main power supply.
On the fuel cell vehicle, when the motor is energized, the voltage across the fuel cell and the voltage across the electricity storage device, which has been increased by the DC/DC converter apparatus, are added together, and the sum voltage is applied to the inverter. When the motor regenerates electric power, the regenerated voltage from the inverter is lowered by the DC/DC converter apparatus and applied to charge the electricity storage device. If the electric power generated by the fuel cell contains an excessive amount of electric power, then it is lowered in voltage and applied to charge the electricity storage device.
FIG. 16 of the accompanying drawings shows a DC/DC converter apparatus 16 disclosed in Japanese Laid-Open Patent Publication No. 2004-357388 that is applied to an electric vehicle. As shown in FIG. 16, the DC/DC converter apparatus 16 basically comprises a DC/DC converter 6 including reactors 2A, 2B, 2C and a switching device comprising three-phase arms made up of upper and lower arm switching devices including transistors 3A, 3B, 3C, 4A, 4B, 4C that are connected inversely across respective diodes 7A, 7B, 7C, 8A, 8B, 8C, and a control means 5 for controlling the DC/DC converter 6.
The DC/DC converter apparatus 16 has a function to convert the voltage of a DC power supply 1 at a low-voltage terminal TL into a voltage that is m times higher and apply the converted voltage to a load 11 through a high-voltage terminal TH (voltage increasing mode), and also a function to convert the voltage at the high-voltage terminal TH into a voltage that is 1/m times lower and apply the converted voltage to the DC power supply 1 through the low-voltage terminal TL (voltage reducing mode).
As shown in FIG. 17 of the accompanying drawings, while the DC/DC converter apparatus 16 is in the voltage increasing mode, when it is driven at a duty ratio of 92 {≈(11/12)×100}[%] in a switching period 2π, the transistors 4A, 4B, 4C of the lower arm switching devices of the three-phase arms are turned on at timings that are 2π/3 out of phase by gate drive signals ULA, ULB, ULC from the control means 5.
While the transistors 4A, 4B, 4C are being energized, since the terminals of the reactors 2A, 2B, 2C which are connected to the respective transistors 4A, 4B, 4C are grounded, the current from the DC power supply 1 flows through the reactors 2A, 2B, 2C to ground. At this time, the reactors 2A, 2B, 2C store an amount of energy which is proportional to the product of the square of the current flowing therethrough and the inductance of the reactors 2A, 2B, 2C.
When the transistors 4A, 4B, 4C are then turned off, a current depending on the energy stored in the reactors 2A, 2B, 2C flows through the diodes 7A, 7B, 7C to the high-voltage terminal TH. The voltage at the high-voltage terminal TH is monitored by a voltage detecting circuit 6a. 
While the DC/DC converter apparatus 16 is in the voltage reducing mode, the transistors 3A, 3B, 3C of the upper arm switching devices are turned on at timings that are 2π/3 out of phase by gate drive signals UHA, UHB, UHC from the control means 5. When the transistors 3A, 3B, 3C are energized, a current flows from the high-voltage terminal TH through the transistors 3A, 3B, 3C and the reactors 2A, 2B, 2C to the DC power supply 1 through the low-voltage terminal TL, storing energy in the reactors 2A, 2B, 2C.
When the transistors 3A, 3B, 3C are then successively turned off, the diodes 8A, 8B, 8C are successively turned on correspondingly, causing a current to flow from ground through the diodes 8A, 8B, 8C and the reactors 2A, 2B, 2C to the DC power supply 1. The DC/DC converter apparatus 16 thus operates as a voltage reducing circuit.
If voltage increasing/reducing DC/DC converter apparatus need to produce an output current higher than the rated current of switching devices such as MOSFETs or IGBTs, then the DC/DC converter apparatus are required to have multiphase arms, rather than a single phase arm, as is the case with the DC/DC converter apparatus 16. Since the output current is distributed to the phase arms, the DC/DC converter apparatus require multiphase reactors, e.g., the three reactors 2A, 2B, 2C for the three phases in the DC/DC converter apparatus 16 shown in FIG. 16.
A reactor has impedance that is greater in proportion to the frequency of a current flowing therethrough. If the output current is constant, then the reactor needs to have a greater inductance as the frequency is lower. Though a reactor with a smaller Q is preferable for a lower resistance loss, a thicker conductive wire is necessary to make a reactor with a smaller Q.
It is desirable that DC/DC converter apparatus be as small and light as possible. However, the need for as many reactors as the number of multiphase arms presents one of obstacles to efforts to reduce the size and weight of the DC/DC converter apparatus.
The DC/DC converter apparatus 16 shown in FIG. 16 employs high-power transistors as switching devices. With the three-phase arms, as shown in plan in FIG. 18A of the accompanying drawings, the six transistors 3A, 3B, 3C, 4A, 4B, 4C are fixedly mounted on a metal heat radiating plate (heat spreader) 12.
As shown in FIG. 17, during the switching period 2π, three transistors are simultaneously energized during a time which is 9/12 of the switching period 2π, and two transistors are simultaneously energized during a remaining time which is 3/12 of the switching period 2π.
FIG. 18B of the accompanying drawings shows a pattern, shown hatched, in which the heat from the transistors is transferred through the heat spreader 12. As shown in FIG. 18B, while the three transistors are simultaneously energized, the heat transferred therefrom concentrates in regions, shown cross-hatched, of the heat spreader 12, resulting in a poor heat radiation efficiency. In order to meet thermal conditions such as maximum allowable temperatures for the transistors, it is necessary to increase the volume and surface area of the heat spreader 12, and to increase the rate of a coolant supplied to cool the heat spreader 12. As a result, the DC/DC converter apparatus 16 tends to be large in size.